What Is a Leap Second and Why Does It Exist?
Source: NIST. Published: 2025-03-15.
Two Ways to Measure a Day
There are fundamentally two ways to track time. The first is astronomical time: measuring the rotation of the Earth against the background of distant stars and the sun. This gives us solar time, which is what clocks traditionally kept — a day was defined by how long the Earth took to rotate once. The second is atomic time: counting the oscillations of atoms, which is extraordinarily stable and reproducible. Since 1967, the second has been defined in terms of the cesium-133 atom, with one second equal to 9 192 631 770 oscillations of the radiation corresponding to a specific transition between energy levels. Atomic clocks are so precise that they would neither gain nor lose one second over tens of millions of years. The problem is that these two timescales — astronomical and atomic — do not stay in step, because the Earth's rotation is gradually slowing down due to tidal friction from the Moon.
UTC and UT1: Two Different Timescales
Coordinated Universal Time (UTC) is the international standard that most clocks and computers use. It runs at the pace of atomic time — identical to International Atomic Time (TAI) in its rate — but is adjusted periodically to stay within 0.9 seconds of UT1, which is a measure of the Earth's actual rotation angle. UT1 matters for applications that require knowledge of where the Earth actually is pointing, such as radio astronomy, satellite navigation, and certain geodetic measurements. If UTC were allowed to diverge from UT1 without correction, navigation systems that depend on the relationship between clock time and Earth's orientation would accumulate errors. The mechanism for keeping UTC and UT1 aligned is the leap second: an extra second inserted into the UTC timescale when the difference between the two approaches 0.9 seconds.
The History of Leap Second Insertions
The leap second system was introduced in 1972, when UTC was first formally defined. Since then, 27 leap seconds have been inserted, all at either June 30 or December 31 at 23:59:59 UTC, making those minutes 61 seconds long. The International Earth Rotation and Reference Systems Service (IERS), based in Paris, monitors the difference between UTC and UT1 and announces leap seconds with about six months' notice. The rate of leap second insertions has varied: they were relatively frequent in the 1970s and 1980s as the atomic timescale accumulated ahead of the slowing Earth, and have been less frequent in recent decades. The most recent leap second was inserted on December 31, 2016. Importantly, leap seconds are not predictable far in advance — the Earth's rotation rate fluctuates irregularly — which means no software can be pre-programmed to handle them automatically.
Why the Computing Industry Hates Leap Seconds
For most people, a leap second passes unnoticed. For distributed computer systems, it is a significant engineering headache. Modern software assumes that a day contains exactly 86 400 seconds and that time always moves forward in a strictly monotonic sequence. A leap second inserts an extra second — 23:59:60 — that most operating systems and programming languages handle poorly or not at all. The standard approach of 'smearing' the leap second across a 24-hour window (slightly slowing clocks before and after the event) is increasingly used by major technology companies but introduces its own inconsistencies for systems that need to agree on exact timestamps. Data centres, financial exchanges, telecommunications networks, and internet infrastructure must all be patched and tested before each leap second, consuming significant engineering resources for a one-second adjustment.
Notorious Leap-Second Incidents
The leap second on June 30, 2012 caused widespread disruptions. Reddit, LinkedIn, Mozilla, and numerous other major websites experienced outages or CPU spikes as their servers — running Linux with the HRTIMER kernel subsystem — encountered a bug triggered by the extra second. Airlines reported flight delay systems freezing. The Qantas check-in system failed for hours. These incidents revealed how deeply the assumption of 86 400 seconds per day was embedded in production software. A later leap second in 2015 caused similar though less severe issues. The pattern demonstrated that even with years of warning and clear documentation, engineering systems at scale have difficulty reliably handling a single extra second — partly because the systems are complex, and partly because leap seconds occur rarely enough that fixes are not thoroughly tested.
The Future: Abandoning Leap Seconds After 2035
In November 2022, the General Conference on Weights and Measures voted to suspend the insertion of leap seconds from 2035, with a view to allowing UTC and UT1 to diverge by more than one second for the first time. The decision was driven largely by pressure from the technology industry and was supported by the recognition that modern timekeeping systems find irregular adjustments deeply problematic. The BIPM and IERS will continue to monitor the divergence; a future decision will determine whether a different correction mechanism (perhaps a 'leap minute' applied once a century) will eventually be introduced. For now, after 2035, UTC will tick forward uniformly with atomic time, and the Earth's rotation — slightly slower than ideal — will simply be tracked separately, its offset from atomic time published and applied only by those applications that genuinely need it.
Use the Timezone Converter